Typical gas sensors designed to measure the concentration of NOx are equipped with an electrode (which will be referred to below as a pump electrode) which works to regulate the oxygen concentration of oxygen-containing measurement gas (exhaust gas) and an electrode (which will be referred to below as a sensor electrode) which works to measure the concentration of NOx in the measurement gas whose oxygen concentration has been regulated. The pump electrode contains Pt (platinum) as a metallic component. The sensor electrode contains Rh (rhodium) as a metallic component in addition to Pt.
For instance, Japanese Patent No. 3701114 teaches a method of avoiding oxidization of a NOx decomposing electrode. The method uses a cermet electrode as the NOx decomposing electrode made of a Pt—Rh alloy and a ceramic component. The cermet electrode has a ratio of Pt to Rh which is selected to be Pt:Rh=10:90 to 50:50 in terms of a weight ratio. Such a ratio of Pt to Rh in the NOx decomposing electrode is described as reducing the oxidization and re-metallization of Rh. Japanese Patent First Publication No. 2003-322634 discloses a NOx decomposing electrode and a NOx concentration measuring device and teaches a weight ratio of Pt to Rh in a cermet electrode layer which is selected to be Pt:Rh=10:90 to 90:10.